The hardness of cubic boron nitride (cBN) is greater than every known material other than diamond. cBN, however, is less reactive with ferrous materials than diamond. Consequently, it has become widely used in material removal applications on ferrous workpieces. Sintered polycrystalline cubic boron nitride (PCBN) materials are well known in the art and widely used in ferrous machining applications.
PCBN compacts can be divided into two broad classes of materials. The first class, which is characterized by the relatively high volume percentage of cBN of greater than or equal to 70%, is widely used in cast iron machining applications. For example, U.S. Pat. No 3,743,489, incorporated herein by reference in its entirety, describes a tough cBN sintered body with direct bonding between cBN grains and metallic binders filling intergranular spaces. The second class is characterized by a lower cBN volume percentage (less than 70%) that is dispersed in a ceramic binder phase. An example of this second type, which is widely used in turning of hardened steels, can be found in U.S. Pat. No. 4,334,928 to Hara et al, incorporated herein by reference in its entirety. The ceramic binder phase can comprise carbides, nitrides, and/or borides of the metals of Groups IVa and Va of the periodic table. Both types of PCBN are commercially available in a solid unsupported form and in tungsten carbide supported form for the fabrication of cutting tools and tool inserts.
Despite the advances of the prior art, some varieties of cast irons, and in particular white irons, remain very difficult and costly materials to machine. In heavy machining operations on massive white iron castings, very large depths of cut are used, up to and exceeding 0.10 inches. These extreme machining conditions require exceptionally tough and wear resistant tool materials. The situation is further complicated by chemical and adhesive wear mechanisms between the tool and workpiece. Many cast irons contain elements that are more reactive than iron against cBN. For example, chromium is found in many white cast irons at levels up to 34 weight-%. This chemical wear mechanism places even more demands upon the tool material.
Currently, high cBN content PCBN in solid form is the most successful class of PCBN material used in heavy machining of cast irons. A commercially available example of such a material is AMB90 (available from Element Six). This material is comprised of about 90 volume-% cBN with 9 μm average grain size in an aluminum ceramic binder. Similarly, U.S. Pat. No. 4,666,466 to Wilson, incorporated herein by reference in its entirety, describes material prepared from a mixture of metallic aluminum and cBN powder reacted under high pressure and temperature conditions to form a sintered body with AlN and AlB2 interspersed between cBN grains. This approach provides a material with relatively good toughness and abrasive wear resistance, but it does not adequately address the need for chemical and adhesive wear resistance that would be required in a truly optimal material for heavy machining of cast irons.
There are a number of references teaching various methods for improving the chemical wear resistance of high cBN content PCBN materials through modification of the binder phases. U.S. Pat. No. 4,343,651 to Yazu et al., incorporated herein by reference in its entirety, describes a PCBN composition with 80-95 volume-% cBN with a particle size of less that 10 μm sintered with a binder phase containing at least one binder material selected from the group consisting of a carbide, a nitride, and a carbonitride of a Group IVb and Vb transition metal in the periodic table, mixtures thereof and their solid solution compounds; and aluminum compounds, wherein the content of aluminum in the matrix is 5 to 30 weight-%, and wherein the particles in the matrix are of a size of less than one micron. U.S. Pat. No. 4,389,465 to Nakai et al., incorporated herein by reference in its entirety, describes a sintered compact consisting essentially of 20-80 volume-% cBN with a particle size less than 5 μm and the residual part consisting of Al2O3. U.S. Pat. No. 4,619,698 to Ueda et al., incorporated herein by reference in its entirety, describes a PCBN material with 75-97 weight-% cBN in a binder phase comprised of 1 to 20 weight % of TiC and/or TiCN and 1 to 20 weight-% of a compound selected from the group consisting of CoAl, NiAl, and (Co, Ni)Al.
As further examples, U.S. Pat. No. 4,911,756 to Nakai et al., incorporated herein by reference in its entirety, describes a sintered compact obtained by sintering a mixture containing about 50 to 75 percent by volume of cBN and about 25 to 50 percent of a binder that contains 20 to 50 weight-% of Al and 4 to 40 weight-% tungsten, with the remainder made up of carbides, nitrides, and carbonitrides of Groups IVa, Va or VIa of the periodic table.
U.S. Pat. No. 5,034,053 to Nakai et al., incorporated herein by reference in its entirety, describes a PCBN material containing 45-75% volume-% of cBN in a binder phase consisting of 5-25 percent by weight of Al and a remainder of at least one species of compounds represented by (H1-Z Mz) C, wherein M is an element selected from the group consisting of Groups IVa, Va and VIa of the periodic table except for Hf, and wherein the condition 0<=z<=0.3 is satisfied. U.S. Pat. No. 5,041,399 to Fukaya et al., incorporated herein by reference in its entirety describes a PCBN material obtained by sintering powder mixture containing 20-70 volume-% cBN and a binder powder comprised of 2-20 weight-% of aluminum, 2-20 weight-% of tungsten, and a remainder formed of one or more Ti compounds selected from a group consisting of TiNz, Ti(C,N)z, TiCz, (Ti,M)Nz, (Ti,M)(C,N)z and (Ti,M)Cz, where M represents a transition metal element or elements belonging to any one of the Groups IVa, Va and VIa of the periodic table excluding Ti, and wherein z is within the range of 0.1 to 0.4, the binder further containing the Al in the form of at least one of Al and a compound of Al and Ti, and the W in the form of at least one of W, WC and a compound of W and Ti, wherein the atomic ratio of the Ti to a transition metal element or metal elements belonging to any one of the Groups, VIa, Va and VIa of the periodic table including Ti is at least ⅔ and not more than 97/100, and wherein cBN crystals are bonded to each other through bonding phases formed by the binder in said sintered body.
U.S. Pat. No. 5,328,875 to Ueda et al., incorporated herein by reference in its entirety, describes a PCBN material wherein cBN grains are dispersed in a binder phase that consists of 20-48 volume-% of a decomposed reaction phase derived by the reaction of cBN and one or more of Ti2-3AlC, Ti2-3AlN, and Ti2-3AlCN including oxygen, the decomposed reaction phase comprising one or more of TiC, TiN, TiCN and one or more of Al2O3 and AlN, and TiB2, and wherein the crystal grain size in the bonding phase and cBN is less than 1 micron.
U.S. Pat. No. 5,830,813 to Yao et al., incorporated herein by reference in its entirety, describes a method for making a PCBN material comprising the steps of: forming a mixture of cBN crystals, a refractory material powder selected from the group consisting of titanium carbonitride and titanium aluminum carbonitride, a source of cobalt and a source of aluminum; treating at least a portion of the ingredients of the mixture with ammonia at a temperature in the range of 1100° to 1250° C.; and sintering the mixture under high temperature, high pressure conditions.
U.S. Pat. No. 6,331,497 to Collier et al., incorporated herein by reference in its entirety, describes a cutting tool comprising a body of polycrystalline cBN with cBN grain size of 10 to 17 μm and a binder phase comprising 2 to 15 weight-% of a material selected from the group consisting of titanium diboride, aluminum diboride, titanium carbide, titanium nitride, titanium carbonitride, titanium aluminum carbonitride, and (TixMy)CN, wherein the alloying metal M may be one or more of silicon, chromium, cobalt, tungsten and tantalum and the proportion of alloying metal to titanium, y/x, is in the range of from 0.05 to 0.3; an infiltrant containing aluminum and/or silicon; and diamond more than stoichiometric with the silicon in the infiltrant for forming silicon carbide.
JP Patent Publication No. 07-082031, incorporated herein by reference in its entirety, discloses a cBN sintered compact consisting of 10-70 volume-% cBN in a binding phase consisting of 5-30 volume-% of alumina (aluminum oxide) with particle diameter of 1 micron or less, 3-20 volume-% of aluminum nitride/boride; 10-40 vol. % of Ti carbide/nitride, and 3-20 volume-% of Ti boride. This sintered compact is reported to be superior to conventional products with longer service life, i.e., improved fracture toughness, thermal shock resistance, chipping resistance, and oxidation resistance.
In JP Patent Publication No. 08-126903, incorporated herein by reference in its entirety, another cBN sintered body with improved wear resistance is disclosed. This cBN sintered body comprises 20-40 volume-% of Ti carbide/nitride, 1-5 volume-% of aluminum nitride, 3-7 volume-% of Ti boride, 3-15 volume-% of aluminum oxide, with the residual consisting of cBN, and wherein at least 60% of the area of the cBN grains are mutually bonded.
JP Patent Publication Nos. 2000-247746A and 2000-218411A, incorporated herein by reference in their entirety, disclose a PCBN material with 30-90 volume-% of a cBN with grain size below 1 μm, 10-70 volume-% of cBN with an average grain size of 2-10 μm, and 4-65 volume-% of a binder phase comprised of AlN, Al2O3, a boride of the metals of Groups IVa, Va, and/or VIa, a non-boride compound from the same groups of the periodic table, and at least one element from Group VII of the periodic table. Although the cBN mixture contains a portion of coarser particles, the volumetric average grain size of the final compact must be around 1 μm.
JP Patent Publication No. 08-109070A, incorporated herein by reference in its entirety, discloses a PCBN material with >85% being comprised of cBN and Al2O3, 30-80 volume-% cBN, and the remainder a binder phase comprised of titanium nitride, aluminum nitride, and unavoidable impurities. The cBN content in the final compact is between 30 and 80 volume-%.
JP Patent Publication No. 59-153851A, incorporated herein by reference in its entirety, discloses a PCBN material obtained by high pressure and temperature sintering of a mixture comprised of 50-70 weight-% cBN, 10-20 weight-% Al2O3, 8-18 weight-% TiN, 8-10 weight-% aluminum powder, and 2-4 weight-% silicon powder.
Thus, while improvements have been realized by the foregoing approaches, there is still a need for an optimized cutting tool material for heavy machining of iron castings. Described below is a PCBN material that has shown improved performance in machining cast irons.